In addition to a biocide, I use a product called Stanadyne. It's a diesel fuel cetane enhancer as well as lubricant. Used primarily by truckers, it seems to work great and is not a "Marine" product, so is relatively inexpensive. Check out the web site. Home - Stanadyne

This is good stuff and recomended by many manufacturers and is popular with the VW and MB crowd.

also Stanadyne is a OE supplier of injectors, and fuel pumps it is designed to not hurt anything in the fuel system.